European patent application published under No. EP 2 008 159 describes a wristwatch provided with a switchover mechanism capable of making one and the same display member selectively indicate the time in one or other of two time zones. The single hours hand selectively displays either the local time or the time in the second time zone, under actuation by a push-button. The switchover mechanism comprises a heart piece and two levers. The time difference between the local time and the time in the second time zone is determined by the relative position of one lever with respect to the other.
The switchover mechanism described in the abovementioned document has the notable disadvantage of including display selection and setting mechanisms which are extremely complex and involve two differentials and an extremely high number of components. In addition, each of the time zones is set using distinct correction geartrains which are independent of the switchover mechanism. It then follows that it is possible to modify one time zone when it is the other time zone that is being displayed.
European patent application published under No. EP 2362277 also describes a timepiece of which the mechanism, under actuation by a push-button, is capable of causing the same hours hand selectively to indicate the time in one or other of two time zones. The device uses a bistable hammer, and two heart piece memory wheels, each one dedicated to a specific time zone, which are kinematically linked within one and the same geartrain which is in mesh with an input of a differential. The time difference between the two time zones is determined by the relative position of the two heart pieces and this is governed by the bistable hammer. Each of the memory wheels also comprises means for setting the time zone which is displayed by the timepiece.
The display setting and switchover mechanisms described in the aforementioned document have the notable disadvantage of being dissociated from the mechanism that sets the hours and minutes of the timepiece. As a result, that device can never in itself be sufficient and has to be combined with a second mechanism so that all of the settings of the timepiece can be carried out. Moreover, that situation demands that there be an additional setting member in addition to the member that sets the time zones, and therefore has impacts on the external parts of the timepiece.